Methods and Apparatus for Wideband Localization

ABSTRACT

A transceiver may wirelessly transmit a communication signal at a first frequency and a sensing signal at a second frequency. The communication signal may include a command that causes a backscatter node to modulate impedance of an antenna, and thereby modulate reflectivity of the backscatter node. The communication signal may also deliver wireless power to the backscatter node. While the impedance is being modulated in response to the command, the transceiver may transmit the sensing signal and measure wireless reflections. The power of the sensing signal may be much lower than that of the communication signal. The transceiver may frequency hop the sensing signal in a wide band of frequencies and take measurements at each frequency in the hopping. Based on the measurements, a computer may determine time-of-flight or phase of a reflected signal from the backscatter node and may estimate location of the backscatter node with sub-centimeter precision.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/936,078 filed on Mar. 26, 2018, which claims the benefit of U.S.Provisional Application No. 62/476,192 filed Mar. 24, 2017 (the“Provisional”).

FIELD OF TECHNOLOGY

The present invention relates generally to wideband localization.

SUMMARY

In illustrative implementations of this invention, the spatialcoordinates of a backscatter node are detected with super-resolution(e.g., less than 1 centimeter) based on time-of-flight (or phase) ofradio signals that reflect from, and are modulated by, the backscatternode. For instance, the backscatter node may comprise an RFID tag, aWi-Fi transceiver, a Bluetooth® transceiver, or a Zigbee® transceiver.Or, for instance, the backscatter node may comprise an RF-energyharvesting sensor employed or any other device that receives power byWPT (wireless power transfer).

In illustrative implementations, this super-resolved localization isachieved even though the bandwidth of the communication band (orwireless power transfer band) of the backscatter node is narrow. Forexample: (a) the backscatter node may be an RFID tag; and (b) the RFIDtag may communicate (and receive wireless power) in an ISM band between902 MHz and 928 MHz. The 26 MHz bandwidth of this ISM band is too narrowto support high precision localization by time-of-flight.

In illustrative implementations, a wide bandwidth (e.g., far wider than26 MHz) is desirable, because time resolution improves as bandwidthincreases. That is, the wider the bandwidth, the better the timeresolution and distance resolution. Put differently, the wider thebandwidth, the smaller the interval of time that can be resolved (andthus the smaller the distance that can be resolved with time-of-flightand the more precise the localization that can be achieved withtime-of-flight).

In illustrative implementations, a wide bandwidth of RF signals isachieved (e.g., by frequency hopping), in order to supportsuper-resolved localization with time-of-flight measurements. Forinstance, the wide bandwidth that is achieved may be greater than orequal to 20 MHz, or greater than or equal to 200 MHz.

In some implementations, a communication signal is employed forcommunication with (and if applicable WPT to) a backscatter node. Thecommunication signal may differ—in frequency and in power—from a sensingsignal employed to localize the backscatter node.

For instance, the communication signal may be at a single frequency thatis in a first, narrow band of frequencies. This first, narrow band offrequencies (for communication or power) may be mandated by governmentregulation. However, the sensing signal may be in a second, much widerband of frequencies that is different than (and at least partiallyoutside of) the first narrow band of frequencies. The sensing signal maybe frequency-hopped or swept, to emulate a wide bandwidth. Or, thesensing signal may be wideband at any given time. In some cases, thecommunication signal has a higher power—or a lower power—than thesensing signal.

For example, an RFID reader may transmit a communication signal tocommunicate with, and to wirelessly power, an RFID tag. Thiscommunication signal may be at a specific frequency in the 902 MHz-928MHz ISM band, and may include an RFID query message.

In this example, the RFID reader may also transmit a sensing signal thatis employed for localizing the RFID tag by time-of-flight measurements.The sensing signal may be frequency hopped (e.g., one frequency at atime) within a band of frequencies that has a bandwidth of at least 200MHz. The RFID reader may measure reflected RF signals (including RFsignals reflecting from the RFID tag) while the sensing signal is ateach different frequency, respectively, in the frequency hopping. Thesemeasurements taken by the RFID reader at different times at differentfrequencies may emulate measurements that would occur if the RFID readerwere to take measurements at a single time during a widebandtransmission.

In this example, the communication signal (for communication and/orpower) may be transmitted at a much higher power than the sensing signal(for localization). The RFID reader may transmit the first signal withinthe ISM band at an EIRP (effective isotropic radiated power) around 36dBm, in order to provide sufficient wireless power to power the RFIDtag. However, the RFID reader may transmit the second signal outside theISM band at an average EIRP of −13.3 dBm, in order to comply with an FCC(Federal Communication Commission) regulation.

In illustrative implementations: (a) a backscatter node modulates thepower of a backscattered signal by rapidly changing impedance in anantenna of the backscatter node; and (b) a transceiver simultaneouslytakes time-of-flight measurements.

In some cases, impedance modulation in the backscatter node causesmodulation of a sensing signal that reflects from the backscatter node.For instance, in some cases, a transceiver transmits a communicationsignal at a first frequency. A backscatter node may respond to thecommunication signal by modulating impedance in the backscatter node,which in turn modulates the RF reflections that reflect from thebackscatter node. While the backscatter node is modulating impedance,the transceiver (e.g., RFID reader) may transmit the sensing signal at asecond frequency. Thus, the impedance modulations in the backscatternode may modulate the power of reflections of the sensing signal thatreflect from the backscatter node.

In some implementations, during the modulation of impedance (in responseto the communication signal), the following processes may occursimultaneously: (1) the transceiver may transmit the sensing signal at aspecific frequency (in the frequency hopping) which is different thanthe frequency of the communication signal and may be outside theconventional communication band of the backscatter node; (2) themodulation of impedance in the backscatter node may modulate reflectionsfrom the backscatter node (including by modulating amplitude or power ofreflections of the sensing signal); and (3) the transceiver may takemeasurements of RF reflections from objects in the transceiver'senvironment, including reflections from the backscatter node.

In some implementations, the modulation of impedance is performed bydedicated hardware in the backscatter node. This modulation of impedancemay in turn modulate the amplitude or power of signals that reflect fromthe backscatter node. For instance, in an RFID tag, the modulation ofimpedance may be performed by a dedicated circuit that includes aswitch. The RFID tag may switch rapidly between: (a) a more reflectivestate and (b) a less reflective state. In the more reflective state, aswitch in the RFID tag may be closed, causing the tag's antenna to beconnected to ground, impedance in a circuit that includes the tag'santenna to be zero (or close to zero), and more RF power to be reflectedby the tag's antenna. In the less reflective state, the switch in theRFID tag may be open, causing RF power to flow into the tag's powerharvesting unit, impedance in a circuit that includes the tag's antennato be high, and less RF power to be reflected by the tag's antenna. Theswitch in the RFID tag that is employed for this modulation (ofimpedance and reflectivity) may comprise a transistor.

Alternatively, in some implementations of this invention, the modulationof impedance in the backscatter node is performed by hardware that isnot dedicated solely to the task of modulating RF reflections from thebackscatter node. For instance, in a Wi-Fi device or Bluetooth® device,a network interface card (NIC) may be rapidly turned off and on, therebycausing a rapid modulation of impedance in the NIC, which in turn causesa rapid modulation of RF signals that passively backscatter from theWi-Fi device or Bluetooth® device. This rapid switching of the state ofthe NIC: (a) may be triggered by a communication signal that istransmitted by a transceiver at a first frequency in the communicationband of the Wi-Fi device or Bluetooth® device; and (b) may occur whilethe transceiver is transmitting a sensing signal at a differentfrequency.

More generally, switching operations may be performed in a backscatternode (e.g., an RFID tag, a Wi-Fi device, a Bluetooth® device, a Zigbee®device, or an RF energy harvesting sensor). These switching operations:(a) may change impedance in the backscatter node and thereby modulatepower of RF signals that reflect from the backscatter node; and (b) maybe in in response to a first signal that is transmitted by a transceiverat a first frequency in the communication band of the backscatter node.Furthermore, these switching operations (and thus modulation ofimpedance and reflected power) may occur while the transceiver istransmitting a second signal at a different frequency which, in manycases, is outside the communication band of the backscatter node. Thetransceiver may frequency hop the second signal and, at each frequencyin the frequency hopping, take measurements of reflected signals,including reflections of the second signal that reflect from, and aremodulated by, the modulation of impedance in the backscatter node.

Thus, in illustrative implementations, a transceiver takes measurementsof reflected signals at different times while the backscatter nodemodulates its impedance and thus modulates reflections from thebackscatter node. For instance, the transceiver may take measurementswhile the transceiver transmits at each different frequency in frequencyhopping of the second signal, respectively, one frequency at a time.

A computer may extract, from these measurements, the second signalreflected from the backscatter node at each of the different frequenciesin the frequency hopping. The computer may determine time-of-flight (orphase) of this second signal, and, based on this time-of-flight (orphase), may determine the 1D, 2D or 3D position of the backscatter node.In some cases, the localization of the backscatter node issuper-resolved. For instance, the system may in some cases detect theposition of the backscatter node with sub-centimeter precision.

The Summary and Abstract sections and the title of this document: (a) donot limit this invention; (b) are intended only to give a generalintroduction to some illustrative implementations of this invention; (c)do not describe all of the details of this invention; and (d) merelydescribe non-limiting examples of this invention. This invention may beimplemented in many other ways. Likewise, the Field of Technologysection is not limiting; instead it identifies, in a general,non-exclusive manner, a field of technology to which someimplementations of this invention generally relate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a high-power signal at 915 MHz striking, and reflectingfrom, an RFID tag.

FIG. 1B illustrates a modulation pattern.

FIG. 1C illustrates a low-power signal at 960 MHz that does not power upthe RFID tag.

FIG. 1D illustrates two signals simultaneously striking, and reflectingfrom, an RFID tag. The first signal is a high-power signal at 915 MHz;the second signal is a low-power signal at 960 MHz.

FIG. 2A shows hardware of an RFID tag, including a switch in the tag.

FIG. 2B shows the RFID tag when the switch is open, impedance is high,and the tag is less reflective.

FIG. 2C shows the RFID when the switch is closed, impedance is low, andthe tag is more reflective.

FIG. 3 illustrates decoupling of localization sensing and wireless powerdelivery.

FIG. 4 shows SNR (signal-to-noise ratio) of an RFID tag's response, as afunction of frequency.

FIG. 5 is a diagram that shows how a high-power signal in the ISM bandand a low-power signal outside the ISM band may be employedsimultaneously in a manner compatible with the EPC Gen2 protocol.

FIG. 6 shows a delay profile.

FIG. 7 shows a method of refining an initial distance measurement, byclustering a set of candidate distances and selecting the cluster withthe lowest WCSS (within-cluster sum of squares).

FIGS. 8A and 8B comprise a flowchart for a method of localizing an RFIDtag.

FIG. 9 shows hardware of a system that is configured to determine thelocation of one or more backscatter nodes.

FIG. 10 is a flowchart for a method of localizing one or morebackscatter nodes.

The above Figures show some illustrative implementations of thisinvention, or provide information that relates to those implementations.The examples shown in the above Figures do not limit this invention.This invention may be implemented in many other ways.

DETAILED DESCRIPTION Overview

In this Detailed Description, we shall first discuss implementations ofthis invention in which the backscatter node is an RFID (radio frequencyidentification) tag. Among other things, we will discuss bandwidth,modulation of impedance, and localization of the RFID tag.

We will also describe a specific RFID prototype and the very accuratelocalization achieved by the prototype.

Then, we shall discuss a wide variety of backscatter nodes which mayemployed in this invention. We will describe how the location of abackscatter node may be determined with great (e.g., sub-centimeter)precision, based on RF signals that reflect from the backscatter node.The backscatter node may comprise, for instance, an RFID tag, a Wi-Fitransceiver, a Bluetooth® transceiver, a Zigbee® transceiver, anRF-energy harvesting sensor, or any other device that is wirelesslypowered.

RFID—Wide Bandwidth/Modulation of Impedance

In some implementations of this invention, the location of RFID tags isaccurately determined, by using a high-power signal at a first frequencyin the 902-928 MHz ISM band for power delivery and communication, and byusing a low-power signal for location sensing. The low-power signal maybe frequency hopped through a wide bandwidth (e.g., at least 200 MHz)thereby emulating a wideband signal. An RFID reader may takemeasurements of reflections of the low-power sensing signal throughoutthis wide bandwidth. Based on these measurements, the time-of-flight (orphase) of the sensing signal may be determined. Based on thetime-of-flight (or phase), location of RFID tags may be determined withgreat accuracy (e.g., sub-centimeter) in each of three x, y, zdimensions. This may be achieved regardless of whether the RFID readeror RFID tag move in a pre-determined path.

This approach—which leverages the benefits of a wide bandwidth—may beperformed with conventional RFID passive tags that are configured toreceive power and communicate in only a narrow band of frequencies (e.g.the 902-928 MHz ISM band). Furthermore, this approach may comply withFCC (Federal Communication Commission) regulations because the low powersignal has a power that is below the maximum power allowed for RFIDsignals outside the ISM band.

The super-resolved localization achieved by this invention has manypractical applications. For example, in some use scenarios, thisinvention enables high-precision localization of on-body RFID tags thattrack multiple limbs of a human user, where the position or movement ofthe limbs comprises an input to control a computer (e.g., to control agame). Furthermore, in some use scenarios, accurate RFID localizationenables employees to check the number of items in a box or whether theright item is in the right box even after the box is sealed. Moregenerally, RFID localization with this invention enables manyapplications in retail stores, factories, warehouses, virtual realityapplications, augmented reality applications, and smart environments.

In a conventional RFID system, an RFID reader transmits a high-powersignal (e.g., 36 dMB) in the ISM frequency band between 902 MHz and 928MHz. In this conventional system: (a) when a passive RFID tag receivesthe high-power signal, the tag powers up by harvesting RF energy fromthe signal; (b) the powered-up tag then modulates impedance in the tag;and (c) this modulation of tag impedance in turn modulates howreflective the tag is, and thus modulates the power of RF (radiofrequency) reflections from the tag.

FIG. 1A shows a conventional RFID system in which a single high-powersignal 110 with a frequency of 926 MHz strikes a passive RFID tag 100.The high-power signal causes tag 100 to power up, and also causes atransistor switch 102 in tag 100 to perform switching that changesimpedance of a circuit connected to the tag's antenna 101. These changesin impedance in turn modulate the power of reflections from the tag.Thus, the backscattered signal 111 is modulated (e.g., in power). InFIG. 1A: (a) backscattered signal 111 is also at 926 MHz; and (b) aconventional RFID reader is transmitting at only a single frequency.

A disadvantage of the conventional approach shown in FIG. 1A is that thetransmitted high-power signal does not have a wide bandwidth, and thuscannot be used for super-resolved localization of the RFID tag. (Recallthat distance resolution may improve—i.e., minimum distance resolvableby the system may become smaller—as bandwidth increases).

FIG. 1B illustrates a modulation pattern 120. In some cases, thismodulation pattern is both: (a) the pattern in which impedance in acircuit that is connected to the tag's antenna is modulated; and (b) theresulting pattern of modulation of reflected power of signals reflectingfrom the tag.

Typically, if an RFID reader were to transmit only a single, very lowpower signal at a frequency outside the ISM 902-928 MHz band, a passiveRFID tag would not power up in response. FIG. 1C illustrates a single,low-power signal 130 at 960 MHz that does not power up the RFID tag. Thepower (e.g., −13.3 dBm) of signal 130 is so low that the signal does notpower up tag 100, and thus tag 100 does not modulate reflections ofsignal 130 from the tag.

In some implementations of this invention, an RFID reader transmits twowireless signals simultaneously: (a) a high-power signal in the ISM bandand (b) a low-power signal that is typically outside the ISM band. Thefirst signal may be high-power (e.g., average EIRP of 36 dBm) and may beat a frequency (e.g., 915 MHz) in the 902-928 MHz ISM band. The firstsignal may be employed for communicating with the tag and for deliveringwireless power to the tag. In response to the first signal, the RFID tagmay power up and then perform switching operations that modulateimpedance in the tag, which in turn modulates how reflective the tag is.The second signal may be extremely low power (e.g., −13.3 dBm) and maybe at a frequency (e.g., 960 MHz) that is outside the 902-928 MHz ISMband. The second signal may be employed for sensing the location of thetag. The modulation of impedance in the tag may, in turn, modulate thepower of reflections (reflecting from the tag) of the first and secondsignals.

FIG. 1D illustrates two signals simultaneously striking, and reflectingfrom, an RFID tag, in an illustrative implementation of this invention.In FIG. 1D, the first signal 110 (and its reflection 111) are each ahigh-power signal at 915 MHz. Likewise, in FIG. 1D, the second signal130 (and its reflection 131) are each a low-power signal at 960 MHz.

In illustrative implementations, localization sensing and wireless powerdelivery are decoupled by transmitting different signals at differentfrequencies. For instance, a transceiver may transmit two wirelesssignals: (a) a first signal at frequency f_(p); and (b) a second signalat frequency f_(s). The first signal may be employed for power deliveryand communication, and the second signal may be employed for sensing.For instance, the first signal f_(p) may be transmitted in the 902MHz-923 MHz ISM band 301 at a high power (e.g., an average EIRP of 36dBm). A passive tag may harvest RF energy from the first signal f_(p).Furthermore, the passive tag and reader may communicate with each othervia the first signal f_(p) (e.g., pursuant to the EPC Gen2 protocol).The second signal f_(s) may be transmitted at a low power (e.g., −13.3dBm) in a frequency hop (e.g., one frequency at a time) over a widerange of frequencies that includes, but is much wider than, the 902-928MHz ISM band. Reflections of the second signal f_(s) may be measured bythe reader, to determine 1D, 2D or 3D spatial coordinates of the reader.

In some implementations, this invention leverages the fact that RFIDmodulation is frequency agnostic—that is, the modulation occurs over awide range of frequencies, including frequencies far outside the 902-928MHz ISM band that is used for RFID power delivery and communication.

In some implementations of this invention, RFID tags communicate with awireless device called an RFID reader through backscatter technology.The reader may transmit a continuous wave at some frequency, and theRFID tag may switch its internal impedance between two states—reflectiveand non-reflective—to communicate bits to the reader. By sensing subtlechanges in the reflected signal due to the tag's impedance changes, anRFID reader may decode the bits communicated by the tag. These impedancechanges may be sensed at various frequencies.

In some implementations of this invention, an RFID reader generates avirtual localization bandwidth that is multiple orders of magnitudelarger than the bandwidth of conventional RFID communication. Ratherthan transmitting a continuous wave at only a single frequency, an RFIDreader may transmit continuous waves at multiple frequencies, as shownin FIG. 1D. When an RFID tag switches its internal impedance to“reflective”, it may reflect all the transmitted frequencies. On theother hand, when it changes its internal impedance to “non-reflective”,it may absorb all of the transmitted frequencies. This may enable theRFID reader to estimate the channel of the RFID tag at all the reflectedfrequencies. A large bandwidth may enable the RFID reader to accuratelycompute time-of-flight (or phase) and use it to localize the RFID tag.

In some implementations, the RFID reader: (a) does not acquire theentire wide bandwidth at once; but (b) instead performs frequencyhopping to emulate a large virtual localization bandwidth in the timedomain. For instance, at every point in time, the RFID reader maytransmit at only two frequencies (one inside the ISM band and anotheroutside the ISM band). Over time, the RFID reader may vary a carrierwave of the sensing frequency and may estimate the channel at thatfrequency. Then, a computer may stitch together the channels at thevarious frequencies obtained from an RFID tag over time. This ispossible because there is no carrier frequency offset (CFO) across timemeasurements since the signals being measured are passive reflections(backscatter) of the reader's signal. Thus, in some implementations ofthis invention, the RFID reader may transmit at a very narrow bandwidthat every point in time and may operate entirely within the bandwidth(and sampling rate) capabilities of RFID readers on the market.

In some implementations, the large virtual localization bandwidth isleveraged to tease apart the various multi-paths in the environment, andidentify the path that arrives earliest in time as the LOS(line-of-sight) path for localization. Then, super-resolved localizationmay be employed to determine, based on the LOS path, the location of theRFID tag.

In some implementations, this invention may employ any type ofbackscatter modulation, including FM0 and Miller-8.

In some implementations, this invention is fully compliant with the RFIDcommunication protocol (the EPC Gen2).

In some implementations, this invention may operate in both LOS(line-of-sight) and NLOS (non-line-of-sight) environments, includinghighly cluttered NLOS environments.

In some implementations, a computer estimates time-of-flight from anRFID reader to an RFID tag, based on measurements of reflected RFsignals taken by an RFID reader. The computer may map the time-of-flightto distance by taking into account the propagation speed of RF signals.To perform 1D localization, one receive antenna may be used. To perform2D or 3D localization, two or three receive antennas respectively may beemployed, and trilateration may be performed.

FIG. 2A shows hardware of an RFID tag. In FIG. 2A, tag 200 includes anantenna 201, a switch 202, a power harvesting unit 203, and a Logics &Memory module 204. Module 204 may include a computer (e.g.,microcontroller) and a memory device.

Typically, a passive RFID tag modulates impedance in a circuit that iselectrically connected to the tag's antenna (and thus the tag modulateshow reflective the tag is) by changing the state of a switch. Forinstance, the switch may comprise a transistor.

FIGS. 2B and 2C, taken together, illustrate how reflectivity of an RFIDtag may be modulated by rapidly switching between open and closed statesof the switch.

FIG. 2B shows RFID tag 200 when switch 202 is open. In FIG. 2B, whenswitch 202 is open: (a) impedance (in a circuit that is electricallyconnected to antenna 201) is high; (b) power flows into power harvestingunit 203; and (c) the tag is less reflective.

FIG. 2C shows the RFID tag 200 when switch 202 is closed. In FIG. 2C,when switch 202 is closed: (a) impedance (in a circuit that iselectrically connected to antenna 201) is low; (b) tag antenna 201 isgrounded; and (c) the tag is more reflective.

In FIGS. 2B and 2C: (a) two signals (s₁ and s₂) are simultaneouslystriking and reflecting from tag 200; (b) the first signal s₁ is ahigh-power (e.g., 36 dBm) signal in the 902-928 MHz IMS band; and (c)the second signal s₂ is a low-power (e.g., −13.3 dBm) signal that may beoutside the 902-928 MHz band.

In some implementations, an RFID reader does not transmit a singlefrequency f₁ as in today's RFID protocol. Instead, the RFID reader maytransmit at multiple frequencies, e.g., f₁ and f₂. When a transistorswitch in the RFID tag is open, the tag absorbs both frequencies asshown in FIG. 2B; when it is closed, the tag reflects both frequenciesas shown in FIG. 2C.

In some implementations, a high SNR (signal-to-noise ratio) is achievedover a wide band of frequencies. This, in turn, allows an RFID reader totake measurements while frequency hopping a sensing signal through awide band of frequencies. As noted above, it is desirable to takemeasurements over a wide band of frequencies, because the wider theband, the better the time resolution and distance resolution.

In FIG. 3, both the first and second signals (at f_(p) and f_(s),respectively) comply with an FCC (Federal Communications Commission)regulation which allows high power for unlicensed RFID communicationswithin ISM band 301 but allows only extremely low power (≤−13.3 dBm)outside of the band. In FIG. 3, line 303 shows the maximum powerpermitted by this FCC regulation, as a function of frequency.

In some implementations, a high SNR (signal-to-noise ratio) is achievedover a wide band of frequencies. This, in turn, allows an RFID reader totake measurements while frequency hopping a sensing signal through awide band of frequencies. As noted above, it is desirable to takemeasurements over a wide band of frequencies, because the wider theband, the better the time resolution and distance resolution.

In some implementations, to sense the channel over a wide bandwidth, theRFID reader may vary f_(s) over time and collect channel measurements.

FIG. 4 shows SNR (signal-to-noise ratio) 403 of an RFID tag's response,as a function of frequency f_(s) of the sensing signal. As shown in FIG.4, the sensing signal has a high SNR over a range of frequencies thatextends far beyond the ISM band. In FIG. 4, the envelope 402 of the SNR403 of the tag's response to the sensing signal f_(s) is greater than 10dB over more than 300 MHz, even though the sensing signal is extremelylow power outside ISM band 401.

RFID—Channel Recovery

In some implementations: (a) an RFID reader measures an RFID tag'sresponse at different sensing frequencies; and (b) a computer recoversthe channels at each of these frequencies. For channel recovery, anychannel estimation technique may be employed.

For instance, in some implementations, a computer may use the knownpreamble p_(t) of the tag's response y_(t) to obtain an estimate of thechannel h_(k) at a given sensing frequency f_(k) as follows:

$\begin{matrix}{h_{k} = {\sum\limits_{t}{y_{t}{p_{t}^{*}}_{\;}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

By repeating this operation over different sensing frequencies, acomputer may obtain channel estimates {h₁ . . . h_(K)} over a widebandwidth.

RFID—EPC Gen2 Protocol

In some implementations of this invention, the signals transmitted bythe RFID reader are compatible with the EPC Gen2 protocol for RFIDcommunication. For instance, an RFID reader may transmit at twofrequencies rather than one at a third stage of a communication session.

FIG. 5 is a diagram that shows how a high-power signal in the ISM bandand a low-power signal outside the ISM band may be employedsimultaneously in a manner that is compatible with the EPC Gen2protocol.

In FIG. 5, during a first time interval 501: (a) an RFID reader maytransmit a high-power signal at frequency f_(p) as a continuous wave;and (b) an RFID tag may simultaneously harvest RF energy from thehigh-power signal. In FIG. 5, frequency f_(p) is within a 902-928 MHzISM band. The tag harnesses power from the signal to power up and startdecoding.

In FIG. 5, during a second time interval 502: (a) the reader maytransmit—as part of the high-power signal at frequency f_(p)—an RFIDquery message; and (b) the tag may receive the query message. The querymessage may provide communication information (e.g., modulation, datarate) to the tag.

After a tag successfully decodes the query, it starts modulating itsantenna impedance to transmit a 16-bit number called RN16. In FIG. 5,during a third time interval 503, the following three things may occursimultaneously: (a) the reader may transmit a high-power signal atfrequency f_(p) as a continuous wave; (b) the reader may also transmit avery low-power signal at frequency f_(s) as a continuous wave; and (c)the RFID tag may, in response to the query, modulate impedance in thetag (and thus modulate amplitude of reflections from the tag) in such away that, when the high-power and low power signals reflect from thetag, their reflections each encode a random 16 bits of data known asRN16.

In FIG. 5, during a fourth time interval 504, the following three thingsmay occur simultaneously: (a) the reader may transmit a high-powersignal at frequency f_(p) as a continuous wave; (b) a computer maydecode the RN16 and estimate the channel of the low power signal atfrequency f_(s); and (c) the tag may harvest RF energy from thehigh-power signal.

In FIG. 5, during a fifth time interval 505: (a) the reader maytransmit, at frequency f_(p), a high-power signal that contains an ACKcommand; (b) the tag may decode the ACK command; and (c) the EPC Gen2protocol may continue.

In some cases: (a) if there is only one tag, then the RN16 may besufficient to identify the reflection from the tag; and (b) if there aremultiple tags, then data transmitted later in the EPC Gen2 protocol maybe employed to uniquely identify a particular tag.

In FIG. 5: (a) the reader may transmit the high-power signal atfrequency f_(p) with an average EIRP of about 36 dBm; and (b) the readermay transmit the low-power signal at frequency f_(s) with an averageEIRP at −13.3 dBm.

The reader may frequency hop the sensing frequency f_(s) in a wide bandof frequencies, and the steps shown in FIG. 5 may be repeated, for eachspecific sensing frequency f_(s) in the frequency hopping. Thus, theRFID reader and tag may repeat the steps shown in FIG. 5 at differentf_(s) carrier waves. In some cases, the RFID reader hops f_(s) over Kconsecutive carriers {f₁, f₂, . . . , f_(K)} as depicted in FIG. 3 wherethe spacing between adjacent carriers is equal to Δf. Alternatively, arandomized hopping pattern or any sweep pattern may be used to transmitat multiple frequencies in a wide band of frequencies.

RFID—Localization

In some implementations of this invention, a computer may perform alocalization algorithm that operates in two stages. First, the algorithmmay tease apart the different paths traversed between an RFID and thereader, and identify the line-of-sight (LOS) path. Second, the algorithmmay “zoom into” the LOS path to achieve sub-centimeter localizationaccuracy.

In indoor environments, RF signals may bounce off different obstacles(such as ceilings, walls, and furniture) before arriving at a receiver.This phenomenon is called the multipath effect.

In some implementations, a computer: (a) analyzes measurements taken byan RFID reader; (b) identifies the LOS path (out of all of the pathsbetween the tag and reader, including LOS and NLOS); and (c) calculatesa rough time-of-flight estimate of that LOS path.

As noted above, the system may obtain channel estimates in the frequencydomain. To identify the LOS path, a computer may transform the channelsfrom the frequency domain to the time domain—i.e., may perform aninverse Fourier transform.

In some cases, in order to identify the LOS path, a computer may performan Inverse Fractional Fourier Transform (IFRFT). An IFRFT isadvantageous, since it incorporates an interpolation mechanism and thusprovides a finer-granularity initial estimate of the time-of-flight.Mathematically, let us denote the channel estimates as h₁, . . . , h_(K)at K different carrier frequencies. To obtain the time domainrepresentation, a computer may perform the following IFRFT operation:

$\begin{matrix}{{S(\tau)} = {\sum\limits_{k = 1}^{K}\;{h_{k}e^{j\; 2\;{\pi{({k - 1})}}\;\Delta\; f\;\tau}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where τ denotes the delay in the time domain, Δf is the frequency stepused in the frequency hopping, and S(τ) is the signal in the timedomain. The solid black line 601 in FIG. 6 shows an example of theoutput of this operation when it is performed over an emulated bandwidthof 220 MHz. FIG. 6 plots the power as a function of the delay τ. Thedelay profile exhibits multiple peaks, each of which corresponds to adifferent path traversed by the signal from the RFID tag.

In illustrative implementations, an initial distance estimate (ofdistance between a reader and a tag) may be calculated from a delayprofile. A computer: (a) may take, as an input, a set of estimatedchannels, where each channel comprises reflection of a low-power signalfrom the tag at a specific frequency in a frequency hop of the low-powersignal; and (b) may perform an Inverse Fourier Transform (e.g., anIFRFT) to calculate a delay profile. The delay profile may describenormalized power as a function of time-of-flight. The computer mayselect the first (in time) large peak in the delay profile, and, basedon the time associated with that large peak, calculate an initialestimate of distance between the reader and the tag. For purposes of thepreceding sentence, a “large” peak may be a peak that is above aspecified constant threshold of normalized power.

FIG. 6 shows a delay profile. In FIG. 6, line 601 is a delay profile,which describes normalized power as a discrete function oftime-of-flight. In FIG. 6: (a) the first large peak 603 in the delayprofile corresponds to an LOS (line-of-sight) reflection from the tag tothe reader; and (b) other large peaks in the delay profile (e.g., 604,605, 606), which occurred later in time, correspond to NLOS(non-line-of-sight) reflections from the tag. (NLOS reflections have alonger time-of-flight because they do not travel directly between thereader and the tag, but instead reflect off or more other objects alongthe way). A computer: (a) may identify the first large peak 603 of thedelay profile (which corresponds to LOS reflections); and (b) initiallyestimate that distance between the reader and tag is equal to thedistance that corresponds to the time-of-flight for the first large peak603.

FIG. 6 also illustrates that a wider bandwidth results in better timeresolution (and thus better distance resolution). In FIG. 6, line 601 isa delay profile calculated by performing an Inverse Fourier Transform ondata derived from measurements taken at different frequencies over awide, 220 MHz band of frequencies. In contrast, in FIG. 6, line 611 is adelay profile calculated by performing an Inverse Fourier Transform ondata derived from measurements taken only within a 26 MHz bandwidth ISMband. In FIG. 6: (a) line 601 (derived from measurements over a wideband) has sufficient time resolution to resolve the different largepeaks (e.g., 603, 604, 605, 6060); but (b) line 611 (derived from only anarrow 26 MHz ISM band) does not.

Thus, FIG. 6 illustrates that, if one has measurements from only a 26MHz bandwidth, it may be difficult or impossible to tease apart the LOSpath from the indirect paths. This is because when the bandwidth issmaller, the different paths may merge into each other. Mathematically,at the output of an Inverse Fourier Transform, each path is convolvedwith a sinc function whose width is inversely proportional to thebandwidth. Specifically, if there are L paths with delays {τ₁ . . .τ_(L)}, we can write the output of the IFRFT as:

$\begin{matrix}{\sum\limits_{i = 1}^{L}{a_{i}sin{c\left( {B\left( {t - \tau_{i}} \right)} \right)}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

where B is the bandwidth and a_(i) is the complex amplitude of thecorresponding path.

Hence, larger B results in fatter sinc functions. In particular, theresolution in separating multipath is the width of the sinc functionmain-lobe, given by:

{Multipath Separability}=1/B  (Equation 4)

Note that the final resolution in estimating each of the paths can bemuch finer.

In FIG. 6, the vertical axis is normalized amplitude. Normalizing theamplitude at different frequencies may be desirable, because the amountof reflection/absorption by an RFID tag may vary over frequency.

As noted above, measurements taken by the RFID reader may be analyzed toidentify the LOS path and provide an initial distance estimate. However,this estimate may be biased by noise and by leakage from other multipathcomponents (e.g., due to the sinc effect described in the textaccompanying Equations 3 and 4)

To refine the initial, coarse estimate of distance, a computer mayleverage phase information. In particular, in the presence of a singleLOS path, the phase ϕ_(k) on the k-th carrier may be written as a directfunction of the distance d:

$\begin{matrix}{\phi_{k} = {\frac{2\pi}{\lambda_{k}}dmod\ 2\pi}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

where λ_(k) is the wavelength for the k-th carrier.

Leveraging this phase, however, is complicated by multiple factors.First, the phase is measured mod 2π; this creates ambiguity in resolvingthe distance (mod λ). Second, the above equation assumes a single LOSpath and ignores both noise and multipath.

To solve these two challenges, two steps may be taken: First, theinitial distance estimate (based on the first large peak in the delayprofile) may be employed as a filter to mitigate the impact of multipathand to recover a phase estimate that may be approximated by Equation 5at each of the frequencies f_(k). Second, an optimization algorithm maybe performed, across the approximate phases calculated at the differentfrequencies, to mitigate the impact of residual noise and leakage. Inwhat follows, we explain these steps in detail.

To recover phase estimates θ_(k) at each of the frequencies f_(k) whilemitigating multipath, a transform may be performed that exploits the LOSestimate of the distance {tilde over (d)}₀ ^(c) as a filter.Specifically, the channels h_(k) at the different frequencies f_(k) maybe projected on the estimate {tilde over (d)}₀ ^(c) of the channelcaused by the LOS path. Such projection may be realized through thefollowing operation:

$\begin{matrix}{\theta_{k} = {\angle{\sum\limits_{i = 1}^{K}\;{h_{k}e^{j\frac{2\;\pi}{c}{({f_{i} - f_{k}})}{\overset{\sim}{d}}_{0}^{c}}}}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

where c is the speed of propagation of the signal.

Effectively, this operation (in Equation 6) may reinforce the signalstrength for the line-of-sight path and may suppress the signal strengthfor the multi-path reflections.

In some implementations, an optimization algorithm may be performed toresolve phase ambiguity. The inputs to this optimization algorithm mayinclude: (a), the filtered phases at different frequencies calculatedpursuant to Equation 6; and (b) the initial distance estimate. (Recallthat an initial distance estimate may be calculated based on the firstlarge peak in a delay profile).

In some implementations, a search for candidate distances is bounded byEquation 4 which limits the potential candidate distances to within asearch range of c/B. Rather than searching over an infinite number ofpotential candidate distances due to the 2π ambiguity of Equation 5, thenumber of potential candidates from each θ_(k) may be:

$\begin{matrix}{{\neq \mspace{14mu}{candidates}} = \frac{c}{B\;\lambda}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

In some use scenarios: (a) frequency hopping of the sensing signalemulates a bandwidth over B=220 MH; and (b) wavelength λ=33 cm; and (b)thus, the search for candidate distances is, per Equation 7, limited tofive candidate distances from each frequency k.

FIG. 7 illustrates an optimization function that may be employed tocreate a more precise (refined) estimate of distance between the RFIDreader and RFID tag. FIG. 7 shows five candidate distances at eachwavelength λ_(k) (or, equivalently, at each frequency f_(k)). Thepotential candidates may be clustered into different groups. Eachcluster may consist of one candidate from each λ_(k), respectively (orequivalently, one candidate for each frequency f_(k), respectively). Thecluster that has the smallest width may correspond to the true location,since it is the one most robust to noise and leakage.

In some use scenarios, a computer selects different clusters C, each ofwhich consists of one distance estimate from each frequency. Then, itselects the cluster that has the minimum within-cluster sum of squares(WCSS), by performing the following optimization function:

$\begin{matrix}{\underset{C}{\arg\mspace{14mu}\min} = {\sum\limits_{d\epsilon C}^{\;}\;{{\hat{d} - \mu}}^{2}}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

where μ is the mean of the distances in the cluster.

The optimization function in Equation 8 may be solved in linear time byexploiting the fact that the unwrapping function is monotonic acrossλ_(k). Said differently, a given candidate at λ_(k+1) may be mapped tothe closest unwrapped candidate at λ_(k).

After identifying the minimum WCSS cluster, a refined, more precisedistance estimate may be expressed as:

{tilde over (d)} ₀ ^(s)=μ  (Equation 9)

Equation 9 estimates distance {tilde over (d)}₀ ^(s) from an RFID tag toa single receiving antenna of an RFID reader. This distance determines acircle in two dimensions and a sphere with a fixed radius in threedimensions. To estimate 2D or 3D location of an RFID tag, two or threereceive antennas, respectively, may be employed and trilateration may beperformed.

We return now to the discussion of FIG. 7. In FIG. 7, a computer selectsa small set of candidate distances (e.g., five candidate distances) foreach frequency, respectively, in the frequency hopping. For example, acomputer may select: (a) five candidate distances (711, 712, 713, 714,715) for frequency λ₁; (b) five candidate distances (721, 722, 723, 724,725) for frequency λ₂; and (c) five candidate distances (731, 732 733,734, 735) for frequency λ_(K). For each frequency, the candidatedistances are all near the initial, course distance estimate 701 (whichwas calculated based on the first large peak in a delay profile, asshown in FIG. 6). In some cases, out of the candidate distances for agiven frequency: (a) at least one candidate distance is greater than orequal to the initial distance estimate 701; and (b) at least onecandidate distance is less than or equal to the initial distanceestimate 701.

In FIG. 7, each candidate distance for a frequency is selected from aset of candidate distances, which are calculated for that frequency inaccordance with Equation 5. (Recall that in Equation 5: (a) phase θ_(k)repeats every 2π, and (b) thus multiple distances correspond to a singlephase θ_(k). Thus, Equation 5 allows one to calculate, for phase θ_(k)of the k^(th) frequency channel, a set of candidate distances.)

In FIG. 7, a computer identifies clusters of candidate distances. Forinstance, cluster 741 includes candidate distances 711, 721, 731 andcluster 742 includes candidate distances 714, 724, 735. (In FIG. 7,cluster 741 also includes a candidate distance for each other frequencybetween λ₂ and λ_(K) in the frequency hopping, and so does cluster 742).In accordance with Equation 8, a computer calculates which cluster hasthe smallest WCSS (within cluster sum of squares). The computer mayestimate that the distance between the RFID tag and a receiving antennaof the RFID reader is equal to the mean of the distances in the clusterthat has the smallest WCSS. This refined estimate may be more accurate(i.e., closer to true distance 703) than is the initial rough estimate.

RFID—Sensing in Complex Domain

In some implementations, the RFID reader senses reflectivity changes inthe complex domain rather than only in the amount of reflection power,as is apparent from the following discussion of RFID backscatter.

The reflected electric field E_(ref) that reflects from an RFID tag maybe expressed as:

E _(ref) ∝E _(inc)×γ  (Equation 10)

where E_(inc) is the electric field incident to the tag.

In Equation 10, γ is a complex number, which can be written as

$\begin{matrix}{\gamma = \frac{R_{a}(f)}{{Z_{a}(f)} + {Z_{c}(f)}^{eff}}} & \left( {{Equation}\mspace{14mu} 11} \right)\end{matrix}$

where (a) Z_(c) ^(eff)(f) is effective chip input impedance, (b)Z_(a)(f) is frequency dependent antenna impedance, (c)Z_(a)(f)=R_(a)(f)+jX_(a)(f), and (d) R_(a)(f) and X_(a)(f) are thefrequency dependent real and imaginary part of antenna impedance,respectively.

The effective chip input impedance is affected by the switchingtransistor. When the switch is open, Z_(c) ^(eff)(f)=Z_(c)(f). When theswitch is closed, Z_(c)≈0. This results in two states of γ:

$\begin{matrix}{\gamma_{open} = {{\frac{R_{a}(f)}{{Z_{a}(f)} + {Z_{c}(f)}}\mspace{14mu}{and}\mspace{14mu}\gamma_{closed}} = \frac{R_{a}(f)}{Z_{a}(f)}}} & \left( {{Equation}\mspace{14mu} 12} \right)\end{matrix}$

An RFID reader may sense the reflected field difference in the complexdomain which may be denoted as

$\begin{matrix}{E_{diff} \propto {E_{in}\left( {\gamma_{closed} - \gamma_{open}} \right)} \propto {E_{in}\frac{{R_{a}(f)}{X_{c}(f)}}{{X_{a}(f)}\left( {{X_{a}(f)} + {X_{c}(f)}} \right)}}} & \left( {{Equation}\mspace{14mu} 13} \right)\end{matrix}$

In some implementations, an RFID reader powers up an RFID tag bytransmitting a high-power signal inside the ISM band, and the tag powersup and switches its impedance. This switching of complex impedance maybe sensed outside the ISM band.

RFID Localization—Illustrative Method

FIGS. 8A and 8B comprise a flowchart for a method of localizing an RFIDtag. In the example shown in FIGS. 8A and 8B, the method includes atleast the following steps: An RFID reader transmits a first wirelesssignal at a first frequency that is within an ISM communication band ofan RFID tag (902 MHz-928 MHz). The RFID tag harvests RF energy from thefirst signal. The first signal may have an average EIRP of approximately36 dBm. The first signal includes a message that triggers the RFID tagto perform switching operations that modulate impedance in the RFID tag.The switching operation may comprise a switch in the RFID tag rapidlychanging between an open and closed state. In the open state of theswitch, the tag's antenna is less reflective, impedance is high, andpower flows into the tag's power harvesting unit. In the closed state ofthe switch, the tag's antenna is more reflective, impedance is low, andthe antenna is grounded (Step 801). While the RFID tag modulatesimpedance in the tag (and thereby modulates power of RF reflections thatreflect from the tag), the RFID reader: (a) transmits a second wirelesssignal at a second frequency; and (b) takes measurements of reflected RFsignals, including reflections of the second signal from the tag. Thesecond signal may be at a much lower power than the first signal (e.g.,−13.3 dBm average EIRP). The second signal reflects from the RFID tagand is modulated, in power, by the modulation of impedance in the tag(Step 802). The RFID reader frequency hops the second signal through awide band of frequencies (e.g., one frequency at a time). For example,the frequency hopping may be in a frequency band that has a bandwidth ofat least 200 MHz and that is entirely or partially outside of the ISMband. At each specific frequency in the frequency hopping, respectively,steps 801 and 802 are repeated with the second frequency being equal tothe specific frequency. This results in a set of measurements taken bythe RFID reader at different times and different frequencies (Step 803).A computer recovers the channels of the second signal at each of thedifferent frequencies (Step 804). A computer calculates an initialdistance estimate, by performing an IFRFT (inverse fractional Fouriertransform, and then selecting a time-of-flight that corresponds to thefirst large peak in the resulting delay profile (Step 805). A computerfilters the second signal in such a way as to reinforce the signalstrength for LOS (line-of-sight) reflections and to reduce the signalstrength for NLOS (non-line-of-sight) multipath reflections. Thisfiltering is performed for each frequency channel of the second signal(Step 806). Based on phase, a computer estimates, for each frequencychannel of the second signal, a small number of candidate distances(between reader and tag) that are close to the initial distance estimate(Step 807). A computer identifies clusters of the candidate distances(one candidate distance per frequency channel), selects the clusterwhich has the smallest WCSS (within-cluster sum of squares), and thencalculates a refined estimate of distance (between reader and tag). Thisrefined estimate of distance is equal to the mean of the distances inthe selected cluster, and is usually more precise (closer to actualdistance) than the initial distance estimate (Step 808). A computer may,based on measurements taken by one antenna of the RFID reader, calculatedistance between reader and tag. A computer may, based on measurementstaken by two antennas of the RFID reader, calculate 2D location of thetag. A computer may, based on measurements taken by three antennas ofthe RFID reader, calculate 3D location of the tag. In each case, thecomputer may employ one or more refined distance estimates from step 808(Step 809).

RFID—Prototype

The following seven paragraphs describe an RFID prototype of thisinvention.

In this prototype, an RFID reader uses two USRPs (universal softwareradio peripherals) with SBX daughterboards. In this prototype: (a) thefirst USRP transmits at 30 dBm at a frequency f_(p) for power deliveryand communication; and (b) the second USRP transmits a sensing frequencyf_(s) at extremely low power (with an average radiation power at −15 dBmand a peak power at −3 dBm and sweeps it over 220 MHz bandwidth. Thesetransmit powers are compliant with FCC regulations for consumerelectronics. The two USRPs are synchronized by an external clock.

In this prototype, to perform 3D localization, an RFID reader uses threeUSRP N210, each with a patch antenna, an external receive chain, and anLFRX daughterboard.

In this prototype, the external receive chain performs coherentdecoding. The receive chain comprises a filter, a variable gain lownoise amplifier (LNA), and an I/Q mixer. The filter eliminates strongleakage from the power delivery carrier f_(p), and helps mitigateself-jamming and reduce phase noise induced by the high-powerself-leakage from f_(p). After filtering, the received signal isamplified by an LNA and down-converted to baseband by mixing with thesensing frequency f_(s) through an I/Q mixer that feeds to an LFRXdaughterboard of the USRP. The USRPs samples baseband I/Q signals whichare postprocessed in MATLAB.

In this prototype, Matlab® software incorporates a Chebyshev-I digitalbandpass filter that rejects residual low-frequency noise and thenperforms matched filtering to recover the channel estimates. A one-timecalibration is performed to account for over-the-wire offsets and forchanges in the reflection coefficient at different frequencies. Theestimated channels are divided by those acquired during the calibrationstep. Then the channels are processed according to the algorithmsdescribed in the above “RFID-Localization” section, to obtain a tag's 3Dlocation.

This prototype was evaluated in a series of tests. These tests wereperformed in multi-path rich indoor environments, including both LOS(line of sight) and NLOS (non-line of sight) settings. Specifically, thetests were performed in an office building with different types ofindoor reflectors including tables, chairs, computers, ceilings, andwalls. The testing environment comprised an office area that is 10meters×12 meters. Localization experiments were performed in multiplesites in this office area against different multipath backgrounds. Theoffice area included office cubicles that were separated by dividersconsisting of 20 cm thick 2 m-tall separators made of two layers ofwood. For NLOS experiments, these separators ensured that there was noLOS path between the reader's antennas and the localized RFIDs. In thesetests: (a) the RFID reader's three receive antennas were separated by 20cm from a transmit antenna of the reader; and (b) the RFID tagscomprised Alien Squiggle™ RFID tags.

A series of 150 tests were performed (in the above environment)regarding the prototype's ability to accurately determine the 2Dlocation of an RFID tag. In these tests, the prototype achieved a medianaccuracy of 0.91 cm and a 90th percentile accuracy of 1.92 cm in 2Dlocalization.

In addition, a series of 160 tests were performed (in the aboveenvironment) regarding the prototype's ability to accurately determinethe 3D location of an RFID tag. Out of these 160 tests, 80 wereperformed LOS settings and 80 were performed in NLOS settings. In thesetests, in both LOS and NLOS settings, the prototype achieved a medianerror that was less than 1.1 cm along each of the x, y, and zdimensions. Moreover, even the 90th percentile error was less than 2 cmin the x and y dimensions, and less than 4 cm in the z dimension. Theaccuracy in LOS was higher than in NLOS settings. Such a result isexpected since the SNR of the line-of-sight path degrades in NLOS,resulting in lower accuracy. (In this paragraph and the precedingparagraph, the “90th percentile error” means a value such that 90percent of the measurements have an error smaller than or equal to thatvalue.)

The prototype described in the preceding seven paragraphs is anon-limiting example of this invention. This invention may beimplemented in many other ways.

Backscatter Nodes, Generally

This invention is not limited to RFID systems.

In illustrative implementations of this invention, the 1D, 2D or 3Dspatial coordinates of a backscatter node are detected based ontime-of-flight (or phase) of radio signals that reflect from, and aremodulated by, the backscatter node.

In illustrative implementations of this invention, the backscatter nodemay be of any type.

In some cases, the backscatter node comprises a device (such as a Wi-Fitransceiver, a Bluetooth® transceiver, or a Zigbee® transceiver) that isitself configured to actively transmit signals in a communication band,and that is also configured to reflect (and modulate) RF signals thatare incident on the device.

In other cases, the backscatter node comprises a device (such as apassive RFID tag) that is not itself configured to actively transmitsignals, but that is configured to reflect (and modulate) RF signalsthat are incident on the device.

In some cases, the backscatter node is configured to harvest RF energy.For example, the backscatter node may comprise a sensor (or an RFID tag)that obtains all or part of its power by harvesting RF energy.

In other cases, the backscatter node is not configured to harvest RFenergy, and instead obtains all its power from other sources such as anelectric power cord or a battery.

In illustrative implementations, the backscatter node is configured tomodulate RF signals that reflect from the backscatter node. Forinstance, the backscatter node may include one or more switches (e.g.,transistors) that are configured to perform switching operations. Theseswitching operations may, in turn, modulate complex impedance in anantenna of the backscatter node (e.g., modulate impedance in a circuitthat is electrically connected to an antenna of the backscatter node).This modulation of impedance may, in turn, modulate the reflectivity ofthis antenna, thereby modulating an RF signal that reflects from thisantenna of the backscatter node.

In some implementations, the modulation of impedance causes thereflectivity of an antenna of the backscatter node to switch repeatedlybetween a first, more reflective state and a second, less reflectivestate. The modulation pattern may be of any type, and of any duty cycle.For instance, the switching between less and more reflective states mayresult in a modulation pattern that approximates a square wave, similarto that shown in FIG. 1B. Or, for instance, the switching between lessand more reflective states may result in a modulation pattern thatapproximates a pulse train, in which the reflectivity is at a constant,single value except during pulses.

In illustrative implementations, a transceiver transmits two wirelesssignals: (a) a communication signal at frequency f_(p); and (b) asensing signal at frequency f_(s). The communication signal may beemployed for communication with (and if applicable, wireless powerdelivery to) the backscatter node. The sensing signal may be employedfor sensing the location of the backscatter node.

In illustrative implementations, the communication signal includes acommand that triggers the backscatter node to perform switchingoperations. These switching operations may modulate the compleximpedance of an antenna in the backscatter node. For instance, if thebackscatter node is an RFID tag, then the communication signal mayinclude a command that directly or indirectly causes a transistor in thetag to switch repeatedly between an open switch state (in which powerflows into the tag's power harvesting unit, impedance is high, and thetag is less reflective) and a closed switch state (in which the tag'santenna is grounded, impedance is low, and the tag is more reflective).Or, for instance, if the backscatter node is a Wi-Fi device, then thecommunication signal may include a command that directly or indirectlycauses an NIC (network interface card) in the Wi-Fi device to switchrepeatedly between an “on” state and an “off” state. These changes instate of the NIC may in turn cause changes in impedance in an antenna ofthe Wi-Fi device. For example, the communication signal may cause amicrocontroller in the Wi-Fi device to output instructions that causethe NIC to switch on and off.

In some implementations, the transceiver transmits the communicationsignal to a backscatter node at a single, constant frequency that iswithin a frequency band that is conventionally employed forcommunication with that type of backscatter node. For instance, in thecase of an RFID tag, the communication signal may be in the 902-928 MHzISM band. Or, for instance, in the case of a Wi-Fi device, thecommunication signal may (in some cases) be at a specific frequency inthe 2.4 GHz-2.5 GHz spectrum and the Wi-Fi device may be configured tocommunicate in a narrow channel that includes that specific frequency.

In illustrative implementations, the transceiver frequency hops thesensing signal through different frequencies in a wide band offrequencies. For example, the wide band of frequencies may have a finitebandwidth of at least 20 MHz. Or, for instance, the wide band offrequencies may have a finite bandwidth that is: (a) at least 100 MHz;(b) at least 200 MHz; (c) at least 300 MHz; (d) at least 400 MHz; (e) atleast 1 GHz; or (f) at least 2 GHz. Or, for instance, the wide band offrequencies may have a bandwidth that is: (a) greater than or equal to20 MHz and less than 100 MHz; (b) greater than or equal to 100 MHz andless than 200 MHz; (c) greater than or equal to 200 MHz and less than300 MHz; (d) greater than or equal to 300 MHz and less than 400 MHz; (e)greater than or equal to 400 MHz and less than 1 GHz; or (f) greaterthan or equal to 1 GHz and less than 2 GHz.

In illustrative implementations, frequency hopping may be performed inany pattern. For instance, frequency hopping may be performed indiscrete, equidistant steps, or may involve random hops. For instance,during the frequency hop, the frequency of the sensing signal: (a): mayvary in discrete steps or continuously; (b) may monotonically increaseor monotonically decrease; (c) may vary in any order, including randomlyor psuedorandomly; or (d) may vary in equidistant steps or innon-equidistant steps. In the frequency hopping, the sensing signal maybe transmitted one frequency at a time. Or, in the frequency hopping,the sensing signal may be transmitted at multiple frequencies at a time.At all times during the frequency hop, the sensing signal may be at adifferent frequency than that of the communication signal.

The transceiver may transmit the sensing signal at a different powerthan the communication signal. In some cases, the sensing signal has alower power than the communication signal. For instance, an RFID readermay transmit the communication signal at an average EIRP of 36 dBm andmay transmit the sensing signal at an average EIRP of −13.3 dBm. In somecases, the transceiver transmits the sensing signal at a power that isequal to, or greater than, the power at which the transceiver transmitsthe communication signal.

The transceiver may transmit the sensing signal while the backscatternode is modulating impedance of an antenna of the backscatter node. (Asnoted above, this modulation may be in response to a command in thecommunication signal).

In some cases, the transceiver: (a) transmits the sensing signal andcommunication signal simultaneously during a portion of a communicationprotocol; and (b) transmits only the communication signal during otherportions of the communication protocol.

Alternatively, in some cases, the transceiver transmits the sensingsignal and communication signal in such a way that: (a) the two signalsalways occur at the same time; or (b) the two signals never occur at thesame time.

In any scenario, the transceiver may transmit the sensing signal at atime when the backscatter node is modulating impedance in response to acommand that occurred earlier in a communication signal.

In some implementations, the sensing signal is a carrier for a thirdsignal. For instance, the third signal (which is carried by a reflectedsensing signal) may comprise the modulation pattern created by changesin impedance in the backscatter node.

In some cases, the sensing signal (as transmitted by the transceiver)comprises a single carrier. In other cases, the sensing signal (astransmitted by the transceiver) comprises a rapidly modulated signalsuch as a Wi-Fi signal or Bluetooth® signal. Modulation by thebackscatter node (e.g., by an RFID tag) during reflection from thebackscatter node may create a relatively slower time-varying envelopewhich may later be decoded.

In illustrative implementations of this invention, the transceiver takesmeasurements of reflected RF signals (including reflections from thebackscatter node), while the transceiver is transmitting each frequencyin the frequency hopping, respectively.

Based on these measurements, a computer may extract a signal from thebackscatter node. For instance, the computer may apply any channelestimation technique to recover a channel at each frequency in thefrequency hopping, respectively.

In some implementations, a computer calculates that the channel for eachspecific frequency of the sensing signal in the frequency hopping,respectively, is equal to a cross-correlation of (i) a known portion(e.g., preamble) of the backscatter node's response and (ii) a signalreceived by the transceiver while the transceiver transmitted thesensing signal at the specific frequency.

In illustrative implementations, a computer calculates, based onmeasurements taken by the transceiver, 1D, 2D or 3D spatial coordinatesof the backscatter. To do so, the computer may perform any localizationalgorithm, including any algorithm that: (a) calculates any absolute orrelative distance or distance metric based on phase or time-of-flight ofa received signal; or (b) calculates, by trilateration or bytriangulation, any absolute or relative distance, distance metric, 2Dspatial coordinate, or 3D spatial coordinate.

In some implementations, a computer computes an initial distanceestimate based on time-of-flight (or phase) along a LOS (line-of-sight)path between the transceiver and the backscatter node.

To do so, a computer may calculate a delay profile, which specifiesnormalized power as a function of time-of-flight. To calculate the delayprofile, a computer may perform an Inverse Fourier Transform (e.g., anInverse Fractional Fourier Transform) that transforms a frequency domainrepresentation of measurements (taken at different frequencies duringthe frequency hop) into the time domain. The normalized powers in thedelay profile may be calculated by dividing each un-normalized power bythe peak power, thereby causing the largest normalized power in thedelay profile to be equal to 1.

In some implementations, a computer may select the first (in time) peakin the delay profile that is above a certain threshold of normalizedamplitude. The computer: (a) may compute a distance that corresponds tothe time-of-flight for this peak; and (b) may set the initial distanceestimate equal to this distance.

Advantageously, in some implementations, the wide bandwidth (in whichfrequency hopping of the sensing signal occurs) allows sufficient timeresolution to separate the peak due to the LOS path (which occurs firstin time) from peaks due to NLOS paths (which occur later in time,because a signal travels further in a NLOS path than in a LOS path).

In some implementations, a computer then calculates a more preciseestimate of distance between the transceiver and backscatter node, againbased on time-of-flight or phase.

To do so, a computer may: (a) calculate, for each frequency f_(k) in thefrequency hopping, a phase θ_(k) of a channel h_(k) of the sensingsignal; (b) filter each channel h_(k) in such a way as to increase thesignal strength due to LOS path and reduce the signal strength due toNLOS paths; (c) calculate, for each frequency f_(k) in the frequencyhopping, a small number of candidate distances (e.g., five candidatedistances) at which the phase of channel h_(k) would be θ_(k); (d)calculate clusters of the candidate distances, in such a way that eachcluster contains one candidate distance for each frequency f_(k),respectively; (e) select the cluster that has the smallest WCSS(within-cluster sum of squares (e.g., this selection may be made inaccordance with Equation 8); and (e) set the more precise estimate ofdistance equal to the mean of the distances in the selected cluster.

In some cases, a computer calculates distance between the backscatternode and each of the transceiver's one or more receive antennas,respectively. The computer may, based on these distances, calculate 1D,2D or 3D position of the backscatter node, by employing trilateration ortriangulation.

For instance, in some cases, 3D position of the backscatter node iscalculated as follows: (a) the transceiver has three receive antennas;(b) the computer calculates, for each receive antenna respectively, asphere that is centered at the receive antenna and that has a radiusequal to the distance between the backscatter node and the receiveantenna; and (c) the computer calculates that the 3D position of thebackscatter node is located where the spheres for the three receiveantennas intersect.

Likewise, in some cases, 2D position of the backscatter node iscalculated as follows: (a) the transceiver has two receive antennas; (b)the computer calculates, for each of receive antenna respectively, acircle that is centered at the receive antenna and that has a radiusequal to the distance between the backscatter node and the receiveantenna; and (c) the computer calculates that the 2D position of thebackscatter node is located where the circles for the two receiveantennas intersect.

Likewise, in some cases, 1D position of the backscatter node iscalculated as follows: (a) the transceiver has one receive antenna; and(b) the computer calculates distance between the backscatter node andthe receive antenna.

In illustrative implementations, the locations of multiple backscatternodes may be determined, based on measurements (taken by thetransceiver) of reflections from these backscatter nodes. For instance,a communication protocol (e.g., an ALOHA protocol) may be employed tosingle out one backscatter node at a time. By detecting one backscatternode at a time, the locations of multiple backscatter nodes may bedetermined. Also, for instance, each of the backscatter nodes may have aunique modulation pattern, and the signals from the differentbackscatter nodes may be disentangled based on these unique modulationpatterns.

FIG. 9 shows hardware of a system that is configured to determine thelocation of one or more backscatter nodes.

In FIG. 9, backscatter nodes 930, 940 may each comprise any kind ofbackscatter node. For instance, backscatter nodes 930, 940 may eachcomprise an RFID tag, a Wi-Fi transceiver, a Bluetooth® transceiver, aZigbee® transceiver, an RF-energy harvesting sensor, or any other devicethat receives power by wireless power transfer.

In FIG. 9, backscatter node 940 includes: (a) antenna 944; (b) one ormore switches (e.g., 941, 942, 943) that are configured to performswitching operations; (c) a computer (e.g., microcontroller) 945, and(d) memory device 946. Likewise, backscatter node 930 includes: (a)antenna 934; (b) one or more switches (e.g., 931, 932, 933) that areconfigured to perform switching operations; (c) a computer (e.g.,microcontroller) 935, and (d) memory device 936.

In FIG. 9, the switching operations performed by the one or moreswitches modulate impedance inside a backscatter node (e.g., impedanceof a circuit that is electrically connected to an antenna of thebackscatter node). Modulating impedance in a backscatter node in turncauses a modulation of the power of RF signals that reflect from thebackscatter node.

In FIG. 9, transceiver 901 transmits a first signal at a first frequencyvia antenna 911 and transmits a second signal at a second frequency viaantenna 912. Transceiver 901 also employs three antennas 914, 915, 916for receiving reflected RF signals. Measurements taken by these threereceive antennas 914, 915, 916 enable computer 921 to determine 3Dcoordinates of a backscatter node. Alternatively, transceiver 901 mayemploy only two antennas for receiving (e.g., 914, 916), and thus enablecomputer 921 to determine 2D coordinates of a backscatter node.Alternatively, transceiver 901 may employ only one antennas forreceiving (e.g., 915), and thus enable computer 921 to determinedistance between transceiver 901 and a backscatter node. In some cases,the number of antennas may be reduced by using one or more circulators.

In FIG. 9, transceiver 901 may frequency hop the second signal (e.g.,one frequency at a time) and may take measurements of RF reflections ateach of the frequencies in the frequency hopping. For a givenbackscatter node, the reflections of the second signal from the givebackscatter node may be modulated (e.g., in power) due to modulation ofimpedance in the given backscatter node. This modulation in impedancemay be due to switching operations that are triggered by the firstsignal.

In FIG. 9, computer 921 may interface with and control transceiver 901.Computer 921 may include a memory device 922. Furthermore, computer 921may interface with or control one or more I/O devices 923, which areconfigured to receive input from a user or to provide output to a user.For instance, I/O devices 923 may include one or more of a touch screen,electronic display screen, mouse, keyboard, microphone, speaker andcamera.

The hardware shown in FIG. 9 may be employed to determine the locationof backscatter nodes 930, 940, by using any localization algorithm.

FIG. 10 is a flowchart for a method of determining the 1D, 2D or 3Dlocation of a backscatter node. In the example shown in FIG. 10, themethod includes at least the following steps: A transceiver transmits afirst wireless signal at a first frequency that is within thecommunication band of a backscatter node. In some cases, the backscatternode harvests RF energy from the first signal. The first signal includesa message that triggers the backscatter node to perform switchingoperations that modulate impedance in the backscatter node (Step 1001).While the backscatter node modulates impedance in the backscatter node(and thereby modulates amplitude of RF reflections that reflect from thebackscatter node), the transceiver: (a) transmits a second wirelesssignal at a second frequency; and (b) takes measurements of reflected RFsignals, including reflections of the second signal from the backscatternode. The second signal may be at a much lower power than the firstsignal. The second signal reflects from the backscatter node and ismodulated, in power, by the modulation of impedance in the backscatternode (Step 1002). The transceiver frequency hops the second signalthrough a wide band of frequencies (e.g., one frequency at a time). Forexample, the frequency hopping may be in a frequency band that has abandwidth of at least 200 MHz and that is entirely or partially outsideof the communication band of the backscatter node. At each specificfrequency in the frequency hopping, respectively, steps 1001 and 1002are repeated with the second frequency being equal to the specificfrequency. This results in a set of measurements taken by thetransceiver at different times and different frequencies (Step 1003). Acomputer extracts, from the measurements, the reflected second signalfrom the backscatter node at the different frequencies of the frequencyhopping. Based on this extracted signal, a computer estimates phase ortime-of-flight of the second signal and calculates 1D, 2D or 3D spatialcoordinates of the backscatter node (Step 1004).

Computers

In illustrative implementations of this invention, one or more computers(e.g., servers, network hosts, client computers, integrated circuits,microcontrollers, controllers, field-programmable-gate arrays, personalcomputers, digital computers, driver circuits, or analog computers) areprogrammed or specially adapted to perform one or more of the followingtasks: (1) to control the operation of, or interface with, hardwarecomponents of a transceiver (e.g., an RFID reader); (2) to control thefrequency and timing of wireless signals transmitted by a transceiver,including to cause the transceiver to transmit a communication (and/orpower) signal at a first frequency and a sensing signal at a secondfrequency; (3) to cause a transceiver to frequency hop the sensingsignal; (4) to cause a transceiver to include a command in thecommunication signal that causes a backscatter node to modulateimpedance in an antenna of the backscatter node (and thereby modulatereflectivity of the backscatter node); (5) to cause the transceiver,while the impedance is being modulated, to transmit the sensing signaland measure reflected RF signals; (6) to extract, from the measurements,reflections from the backscatter node, such as by performing any channelestimation technique; (7) to estimate a channel for each frequency inthe frequency hopping, (8) to perform any localization algorithm; (9) toreceive data from, control, or interface with one or more sensors; (10)to perform any other calculation, computation, program, algorithm, orcomputer function described or implied herein; (11) to receive signalsindicative of human input; (12) to output signals for controllingtransducers for outputting information in human perceivable format; (13)to process data, to perform computations, and to execute any algorithmor software; and (14) to control the read or write of data to and frommemory devices (tasks 1-14 of this sentence referred to herein as the“Computer Tasks”). The one or more computers (e.g. 921) may, in somecases, communicate with each other or with other devices: (a)wirelessly, (b) by wired connection, (c) by fiber-optic link, or (d) bya combination of wired, wireless or fiber optic links.

In exemplary implementations, one or more computers are programmed toperform any and all calculations, computations, programs, algorithms,computer functions and computer tasks described or implied herein. Forexample, in some cases: (a) a machine-accessible medium has instructionsencoded thereon that specify steps in a software program; and (b) thecomputer accesses the instructions encoded on the machine-accessiblemedium, in order to determine steps to execute in the program. Inexemplary implementations, the machine-accessible medium may comprise atangible non-transitory medium. In some cases, the machine-accessiblemedium comprises (a) a memory unit or (b) an auxiliary memory storagedevice. For example, in some cases, a control unit in a computer fetchesthe instructions from memory.

In illustrative implementations, one or more computers execute programsaccording to instructions encoded in one or more tangible,non-transitory, computer-readable media. For example, in some cases,these instructions comprise instructions for a computer to perform anycalculation, computation, program, algorithm, or computer functiondescribed or implied herein. For example, in some cases, instructionsencoded in a tangible, non-transitory, computer-accessible mediumcomprise instructions for a computer to perform the Computer Tasks.

Network Communication

In illustrative implementations of this invention, electronic devices(e.g., 901, 921) are configured for wireless or wired communication withother devices in a network.

For example, in some cases, one or more of these electronic devices eachinclude a wireless module for wireless communication with other devicesin a network. Each wireless module may include (a) one or more antennas,(b) one or more wireless transceivers, transmitters or receivers, and(c) signal processing circuitry. Each wireless module may receive andtransmit data in accordance with one or more wireless standards.

In some cases, one or more of the following hardware components are usedfor network communication: a computer bus, a computer port, networkconnection, network interface device, host adapter, wireless module,wireless card, signal processor, modem, router, cables or wiring.

In some cases, one or more computers (e.g., 921) are programmed forcommunication over a network. For example, in some cases, one or morecomputers are programmed for network communication: (a) in accordancewith the Internet Protocol Suite, or (b) in accordance with any otherindustry standard for communication, including any USB standard,ethernet standard (e.g., IEEE 802.3), token ring standard (e.g., IEEE802.5), wireless standard (including IEEE 802.11 (wi-fi), IEEE 802.15(bluetooth/zigbee), IEEE 802.16, IEEE 802.20 and including any mobilephone standard, including GSM (global system for mobile communications),UMTS (universal mobile telecommunication system), CDMA (code divisionmultiple access, including IS-95, IS-2000, and WCDMA), or LTS (long termevolution)), or other IEEE communication standard.

Definitions

The terms “a” and “an”, when modifying a noun, do not imply that onlyone of the noun exists. For example, a statement that “an apple ishanging from a branch”: (i) does not imply that only one apple ishanging from the branch; (ii) is true if one apple is hanging from thebranch; and (iii) is true if multiple apples are hanging from thebranch.

Backscatter node” means an object that backscatters a radio signal.Non-limiting examples of a “backscatter node” include: (a) an RFID tagthat backscatters a radio signal; and (b) an object that backscatters aradar signal.

To say that a calculation is “according to” a first equation means thatthe calculation includes (a) solving the first equation; or (b) solvinga second equation, where the second equation is derived from the firstequation. Non-limiting examples of “solving” an equation include solvingthe equation in closed form or by numerical approximation or byoptimization.

To compute “based on” specified data means to perform a computation thattakes the specified data as an input.

The term “comprise” (and grammatical variations thereof) shall beconstrued as if followed by “without limitation”. If A comprises B, thenA includes B and may include other things.

The term “computer” includes any computational device that performslogical and arithmetic operations. For example, in some cases, a“computer” comprises an electronic computational device, such as anintegrated circuit, a microprocessor, a mobile computing device, alaptop computer, a tablet computer, a personal computer, or a mainframecomputer. In some cases, a “computer” comprises: (a) a centralprocessing unit, (b) an ALU (arithmetic logic unit), (c) a memory unit,and (d) a control unit that controls actions of other components of thecomputer so that encoded steps of a program are executed in a sequence.In some cases, a “computer” also includes peripheral units including anauxiliary memory storage device (e.g., a disk drive or flash memory), orincludes signal processing circuitry. However, a human is not a“computer”, as that term is used herein.

“Defined Term” means a term or phrase that is set forth in quotationmarks in this Definitions section.

For an event to occur “during” a time period, it is not necessary thatthe event occur throughout the entire time period. For example, an eventthat occurs during only a portion of a given time period occurs “during”the given time period.

The term “e.g.” means for example.

“EIRP” means effective isotropic radiated power.

Each equation above is referred to herein by the equation number setforth to the right of the equation. For example: “Equation 1” meansEquation 1 above; and “Equation 6” means Equation 6 above. Non-limitingexamples of an “equation”, as that term is used herein, include: (a) anequation that states an equality; (b) an inequation that states aninequality (e.g., that a first item is greater than or less than asecond item); (c) a mathematical statement of proportionality or inverseproportionality; and (d) a system of equations.

The fact that an “example” or multiple examples of something are givendoes not imply that they are the only instances of that thing. Anexample (or a group of examples) is merely a non-exhaustive andnon-limiting illustration.

Unless the context clearly indicates otherwise: (1) a phrase thatincludes “a first” thing and “a second” thing does not imply an order ofthe two things (or that there are only two of the things); and (2) sucha phrase is simply a way of identifying the two things, respectively, sothat they each may be referred to later with specificity (e.g., byreferring to “the first” thing and “the second” thing later). Forexample, unless the context clearly indicates otherwise, if an equationhas a first term and a second term, then the equation may (or may not)have more than two terms, and the first term may occur before or afterthe second term in the equation. A phrase that includes a “third” thing,a “fourth” thing and so on shall be construed in like manner.

“For instance” means for example.

To say a “given” X is simply a way of identifying the X, such that the Xmay be referred to later with specificity. To say a “given” X does notcreate any implication regarding X. For example, to say a “given” X doesnot create any implication that X is a gift, assumption, or known fact.

A non-limiting example of a device “harvesting RF energy” is the deviceobtaining at least a portion of its power by harvesting RF energy.

“Herein” means in this document, including text, specification, claims,abstract, and drawings.

As used herein: (1) “implementation” means an implementation of thisinvention; (2) “embodiment” means an embodiment of this invention; (3)“case” means an implementation of this invention; and (4) “use scenario”means a use scenario of this invention.

The term “include” (and grammatical variations thereof) shall beconstrued as if followed by “without limitation”.

“ISM radio frequency band” means any of the following frequency bands:(a) 6.765 MHz to 6.795 MHz; (b) 13.553 MHz to 13.567 MHz; (c) 26.957 MHzto 27.283 MHz; (d) 40.66 MHz to 40.7 MHz; (e) 433.05 MHz to 434.79 MHz;(f) 902 MHz to 928 MHz; (g) 2.4 GHz to 2.5 GHz; (h) 5.725 GHz to 5.875GHz; (i) 24 GHz to 24.25 GHz; (j) 61 GHz to 61.5 GHz; (k) 122 GHz to 123GHz; and (1) 244 GHz to 246 GHz.

The term “or” is inclusive, not exclusive. For example, A or B is trueif A is true, or B is true, or both A or B are true. Also, for example,a calculation of A or B means a calculation of A, or a calculation of B,or a calculation of A and B.

A parenthesis is simply to make text easier to read, by indicating agrouping of words. A parenthesis does not mean that the parentheticalmaterial is optional or may be ignored.

“Power delay profile” means a set of data that specifies power (or avalue that is derived from power) as a function of time. A non-limitingexample of a “power delay profile” is a set of data that specifiesnormalized power as a function of time-of-flight.

“Radio energy harvesting device” means a device that is configured toharvest energy from wireless radio signals. Non-limiting examples of a“radio energy harvesting device” include: (a) a device that isconfigured to harvest energy from wireless radio signals and from othersources; and (b) a device that is configured to be partially powered bywireless radio signals.

A “reading” means a measurement.

“RF” means radio frequency.

As used herein, the term “set” does not include a group with noelements.

A “set of” different values of X means a set of values of X, each valuein the set being different than each other value in the set.

Unless the context clearly indicates otherwise, “some” means one ormore.

As used herein, a “subset” of a set consists of less than all of theelements of the set.

To “sweep” a signal means to transmit the signal in such a way thatfrequency of the signal changes over time. Non-limiting examples of“sweeping” a signal include: (a) varying frequency of the signal indiscrete steps or continuously; (b) monotonically increasing ormonotonically decreasing frequency of the signal; (c) varying frequencyof the signal in any order, including in a pseudorandom sequence offrequencies; and (d) varying frequency of the signal in equidistantsteps or in non-equidistant steps.

The term “such as” means for example.

“Time-of-flight between” X and Y means a length of time that a wirelessradio signal would take: (a) to travel from X to Y; or (b) to travel ina roundtrip that consists of traveling from X to Y and then travelingfrom Y to X. A non-limiting example of “time-of-flight between” X and Yis a length of time that a wireless radio signal would take to travelfrom X to Y in a straight path. Another non-limiting example of“time-of-flight between” X and Y is a length of time that a wirelessradio signal would take to travel from X to Y in an indirect path inwhich the signal reflects from one or more objects while traveling fromX to Y.

A transceiver is a non-limiting example of a “receiver”.

“Transceiver” means a device that includes both a transmitter and areceiver. Nonlimiting examples of a transceiver include: (a) a device inwhich a transmitter and a receiver share common circuitry; (b) a devicethat houses both a transmitter and a receiver in a single housing; or(c) a device that includes both a transmitter and a receiver, whereinthe transmitter and transceiver do not share common circuitry and arenot housed together in a single housing.

To say that a machine-readable medium is “transitory” means that themedium is a transitory signal, such as an electromagnetic wave.

A non-limiting example of an event that occurs “while changes inimpedance are occurring” is an event that occurs during a temporalsequence of changes in impedance in a backscatter node, which sequenceis triggered by a command, even though the event occurs between twochanges of impedance in the sequence. Likewise, a non-limiting exampleof an event that occurs “at a time at which changes in impedance areoccurring” is an event that occurs during a temporal sequence of changesin impedance in a backscatter node, which sequence is triggered by acommand, even though the event occurs between two changes of impedancein the sequence.

Unless the context clearly indicates otherwise, “while A and B” meanswhile A and B are both occurring.

“Wireless command” means a signal that is wirelessly transmitted andthat encodes a command.

Except to the extent that the context clearly requires otherwise, ifsteps in a method are described herein, then the method includesvariations in which: (1) steps in the method occur in any order orsequence, including any order or sequence different than that describedherein; (2) any step or steps in the method occurs more than once; (3)any two steps occur the same number of times or a different number oftimes during the method; (4) any combination of steps in the method isdone in parallel or serially; (5) any step in the method is performediteratively; (6) a given step in the method is applied to the same thingeach time that the given step occurs or is applied to different thingseach time that the given step occurs; (7) one or more steps occursimultaneously, or (8) the method includes other steps, in addition tothe steps described herein.

Headings are included herein merely to facilitate a reader's navigationof this document. A heading for a section does not affect the meaning orscope of that section.

This Definitions section shall, in all cases, control over and overrideany other definition of the Defined Terms. The Applicant or Applicantsare acting as his, her, its or their own lexicographer with respect tothe Defined Terms. For example, the definitions of Defined Terms setforth in this Definitions section override common usage or any externaldictionary. If a given term is explicitly or implicitly defined in thisdocument, then that definition shall be controlling, and shall overrideany definition of the given term arising from any source (e.g., adictionary or common usage) that is external to this document. If thisdocument provides clarification regarding the meaning of a particularterm, then that clarification shall, to the extent applicable, overrideany definition of the given term arising from any source (e.g., adictionary or common usage) that is external to this document. To theextent that any term or phrase is defined or clarified herein, suchdefinition or clarification applies to any grammatical variation of suchterm or phrase, taking into account the difference in grammatical form.For example, the grammatical variations include noun, verb, participle,adjective, and possessive forms, and different declensions, anddifferent tenses.

VARIATIONS

This invention may be implemented in many different ways. Here are somenon-limiting examples:

In some implementations, this invention is an apparatus that comprises:(a) a transceiver; and (b) one or more computers, wherein (i) thetransceiver is configured (A) to wirelessly transmit a first radiosignal at a first frequency, the first signal encoding a command, (B) towirelessly transmit a second radio signal at a second frequency, thesecond frequency being different than the first frequency, and (C) totake measurements of reflections of the second radio signal, includingreflections from a backscatter node, while (I) the apparatus istransmitting the second radio signal at the second frequency and (II)changes in impedance are occurring, in response to the command, in thebackscatter node, and (ii) the one or computers are programmed toextract, from the measurements, a signal from the backscatter node. Insome cases, the transceiver is configured to transmit the second signalat an average EIRP that is less than or equal to −13.3 dBm. In somecases, the first frequency is in an ISM radio frequency band. In somecases, the second signal is a Wi-Fi signal. In some cases, the secondsignal is a Bluetooth signal. In some cases, the second signal is aZigbee signal. In some cases, the transceiver is configured towirelessly transmit the second signal in such a way that the secondsignal, as transmitted by the transceiver, is a carrier signal thatcarries a third signal. In some cases, the transceiver is configured towirelessly transmit the first signal at the first frequency andsimultaneously to wirelessly transmit the second signal at the secondfrequency. In some cases, the one or computers are programmed tocalculate, based on the measurements, a location of the backscatternode. In some cases: (a) the transceiver is configured to sweep thesecond frequency and take a set of readings during the sweep in such away that (i) the second frequency changes during the sweep, and (ii) foreach value, respectively, in a set of different values of the secondfrequency during the sweep, the transceiver takes the actions describedin clause (i) of the first sentence of this paragraph; and (b) the oneor computers are programmed to calculate, based on the set of readings,a location of the backscatter node. In some cases, the one or computersare programmed to calculate, based on the set of readings, a power delayprofile. In some cases, the one or more computers are programmed: (a) toidentify, in the power delay profile, a specific peak of normalizedpower; and (b) to estimate, for the specific peak, a time-of-flightbetween the transceiver and the backscatter node. In some cases: (a) thepower delay profile includes a set of peaks of normalized power,including the specific peak, that are each above a specific threshold ofnormalized power; (b) normalized power at each of the peaks,respectively, is a function of time-of-flight; and (c) thetime-of-flight for the specific peak is smaller than the time-of-flightfor each other peak, respectively, in the set of peaks. In some cases:(a) the power delay profile includes a set of peaks of normalized power,including the specific peak, that are each above a specific threshold ofnormalized power; and (b) the normalized power for the specific peak isgreater than the normalized power for each other peak, respectively, inthe set of peaks. In some cases: (a) the second frequency is a member ofa set of frequencies, each frequency in the set of frequencies beingdifferent than the first frequency; and (b) the apparatus is configuredin such a way that for each specific frequency in the set offrequencies, respectively, the apparatus wirelessly transmits the secondradio signal at the specific frequency at a time at which the changes inimpedance are occurring. In some cases, the bandwidth of the set offrequencies is at least 20 MHz. In some cases, the one or computers areprogrammed to estimate phase of the second signal for each frequency inthe set of frequencies, respectively. Each of the cases described abovein this paragraph is an example of the apparatus described in the firstsentence of this paragraph, and is also an example of an embodiment ofthis invention that may be combined with other embodiments of thisinvention.

In some implementations, this invention is an apparatus that comprises:(a) a receiver; and (b) one or more computers, wherein (i) the receiveris configured to take measurements of reflections of a radio signalwhile (A) the radio signal is at a first frequency, and (B) abackscatter node is changing impedance in response to a wirelesscommand, the wireless command being at a different frequency than thefirst frequency, and (ii) the one or more computers are programmed toextract, from the measurements, a signal from the backscatter node. Insome cases, the one or more computers are programmed to calculate, basedon the signal from the backscatter node, one-dimensional,two-dimensional or three-dimensional spatial coordinates of thebackscatter node. Each of the cases described above in this paragraph isan example of the apparatus described in the first sentence of thisparagraph, and is also an example of an embodiment of this inventionthat may be combined with other embodiments of this invention.

In some implementations, this invention is an apparatus that comprises:(a) a transceiver; and (b) one or more computers, wherein (a) thetransceiver is configured to simultaneously (i) transmit a first radiosignal at a first frequency, (ii) sweep a second radio signal through aband of frequencies, which band includes frequencies other than thefirst frequency, and (iii) take measurements of reflections of thesecond signal, which reflections include reflections that reflect from aradio frequency energy harvesting device that harvests energy from thefirst signal; and (b) the one or more computers are programmed tocalculate, based on the measurements, one-dimensional, two-dimensionalor three-dimensional spatial coordinates of the energy harvestingdevice. The embodiment of this invention that is described is thisparagraph may be combined with other embodiments of this invention.

Each description herein (or in the Provisional) of any method, apparatusor system of this invention describes a non-limiting example of thisinvention. This invention is not limited to those examples, and may beimplemented in other ways.

Each description herein (or in the Provisional) of any prototype of thisinvention describes a non-limiting example of this invention. Thisinvention is not limited to those examples, and may be implemented inother ways.

Each description herein (or in the Provisional) of any implementation,embodiment or case of this invention (or any use scenario for thisinvention) describes a non-limiting example of this invention. Thisinvention is not limited to those examples, and may be implemented inother ways.

Each Figure herein (or in the Provisional) that illustrates any featureof this invention shows a non-limiting example of this invention. Thisinvention is not limited to those examples, and may be implemented inother ways.

The above description (including without limitation any attacheddrawings and figures) describes illustrative implementations of theinvention. However, the invention may be implemented in other ways. Themethods and apparatus which are described herein are merely illustrativeapplications of the principles of the invention. Other arrangements,methods, modifications, and substitutions by one of ordinary skill inthe art are also within the scope of the present invention. Numerousmodifications may be made by those skilled in the art without departingfrom the scope of the invention. Also, this invention includes withoutlimitation each combination and permutation of one or more of theimplementations (including hardware, hardware components, methods,processes, steps, software, algorithms, features, or technology) thatare described herein.

What is claimed:
 1. A method comprising: (a) wirelessly transmitting afirst radio signal at a first frequency, the first signal encoding acommand; (b) wirelessly transmitting a second radio signal at a secondfrequency, the second frequency being different than the firstfrequency; (c) taking measurements of reflections of the second radiosignal, including reflections from a backscatter node, while (I) theapparatus is transmitting the second radio signal at the secondfrequency and (II) changes in impedance are occurring, in response tothe command, in the backscatter node; and (d) extracting, from themeasurements, a signal from the backscatter node.
 2. The method of claim1, wherein the second signal is transmitted at an average EIRP that isless than or equal to −13.3 dBm.
 3. The method of claim 1, wherein thefirst frequency is in an ISM radio frequency band.
 4. The method ofclaim 1, wherein the second signal is a Wi-Fi signal.
 5. The method ofclaim 1, wherein the second signal is a Bluetooth signal.
 6. The methodof claim 1, wherein the second signal is a Zigbee signal.
 7. The methodof claim 1, wherein the second signal is a carrier signal that carries athird signal.
 8. The method of claim 1, wherein the first signal istransmitted at the first frequency and simultaneously the second signalis transmitted at the second frequency.
 9. The method of claim 1,wherein the method further comprises calculating, based on themeasurements, a location of the backscatter node.
 10. The method ofclaim 1, wherein the method further comprises: (a) performing a sweep ofthe second frequency and take a set of readings during the sweep in sucha way that (i) the second frequency changes during the sweep, and (ii)for each value, respectively, in a set of different values of the secondfrequency during the sweep, taking the actions described in clause (i)of claim 1; and (b) calculating, based on the set of readings, alocation of the backscatter node.
 11. The method of claim 10, whereinthe method further comprises calculating, based on the set of readings,a power delay profile.
 12. The method of claim 11, wherein the methodfurther comprises: (a) identifying, in the power delay profile, aspecific peak of normalized power; and (b) estimating, for the specificpeak, a time-of-flight between the transceiver and the backscatter node.13. The method of claim 12, wherein: (a) the power delay profileincludes a set of peaks of normalized power, including the specificpeak, that are each above a specific threshold of normalized power; (b)normalized power at each of the peaks, respectively, is a function oftime-of-flight; and (c) the time-of-flight for the specific peak issmaller than the time-of-flight for each other peak, respectively, inthe set of peaks.
 14. The method of claim 12, wherein: (a) the powerdelay profile includes a set of peaks of normalized power, including thespecific peak, that are each above a specific threshold of normalizedpower; and (b) the normalized power for the specific peak is greaterthan the normalized power for each other peak, respectively, in the setof peaks.
 15. The method of claim 1, wherein: (a) the second frequencyis a member of a set of frequencies, each frequency in the set offrequencies being different than the first frequency; and (b) for eachspecific frequency in the set of frequencies, respectively, the secondradio signal is wirelessly transmitted at the specific frequency at atime at which the changes in impedance are occurring.
 16. The method ofclaim 15, wherein the bandwidth of the set of frequencies is at least 20MHz.
 17. The method of claim 15, wherein the method further comprisesestimating phase of the second signal for each frequency in the set offrequencies, respectively.